OTTAWA (Dow Jones) -- The U.S. has lost a key round at the World Trade Organization in a trade dispute with Canada and Mexico over meat labeling, according to people familiar with the WTO's findings.

Canada and Mexico opposed a new U.S. rule that requires more information on labels about the origins of beef, pork and other meats, which went into effect in November. They took their case to the WTO, saying the rule hurts their competitiveness. The WTO panel that heard oral arguments in the dispute over the so-called country-of-origin labeling rule earlier this year has decided in favor ...